Mouse running amok

Is someone controlling my computer?

Probably not, but to be on the safe side, you can unplug your network connection and see if it still happens.

I suspect your mouse is just broken.

If you want to be absolutely sure:

• You can check what applications are currently connected to the network in some way; Open a Terminal (Applications → Accessories → Terminal) and type:

  sudo lsof -i
  

  this will prompt you for your password* and list a bunch of processes, you can see what they are by typing man command, i.e. man master and see if they look trustworthy.

  Note that in order to control the mouse, the process would need to belong to the "root" user, you can ignore all the other ones.

  these are mine:

  stefano@3000-G530:~$ sudo lsof -i
  [sudo] password for stefano: 
  COMMAND     PID    USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
  cupsd      1068    root    5u  IPv6 3663430      0t0  TCP 3000-G530:ipp (LISTEN)
  cupsd      1068    root    7u  IPv4 3663431      0t0  TCP localhost.localdomain:ipp (LISTEN)
  hddtemp    1492    root    0u  IPv4   10406      0t0  TCP localhost.localdomain:7634 (LISTEN)
  master     1597    root   12u  IPv4   10768      0t0  TCP localhost.localdomain:smtp (LISTEN)
  dhclient   9178    root    5u  IPv4 3012776      0t0  UDP *:bootpc 
  

  Everything is normal here, I know what all of these do. If you're not sure about one of the processes, post a comment. But As I said, it would really surprise me if there was anything nasty going on. You are very well protected with a firewall that has no open ports at all by default.

*: When you type your password, you don't see any characters: this is a security feature, not an error.

If it is an optical mouse, then it is quite likely that yours drank cyanide and is kicking the bucket.

In other words, it is most likely a hardware failure. As Stefano said, unplug your network cable and see it it happens again. You can also unplug the mouse and see if it happens. If it happens with the mouse out and the network cable in, then you might have an intruder, and you would need to check and see where your system is open to attack (for that, would be best to open a new question). If it happens with both mouse and network cable out - ghosts? O.o...

I use wireless mice and most of my mouse problems are fixed by two things:

1) Replace the batteries (when there are some). Sometimes mice stop working on low power, sometime they act very strange, similar to what you describe.

2) Remove and re-insert the USB dongle. I was skeptical when I first read this, so was pleasantly surprised when I find it is a frequent fix to problems (especially super-fast scrolling up/down issues).

These are specific to wirless mouse but as they are extremely common these days I post this to help others who experience these problems and have wireless mice.